In oil-producing fields employing waterflooding, the water tends to channel through zones of high permeability and to by-pass a large amount of oil-in-place. Since waterflooding fluids usually contain chemicals, and even relatively small amounts of chemicals in the hundreds of millions of gallons of water employed become expensive, an efficient sweep by the waterflood is important.
Water channeling can be reduced by injecting a solution of a polymer and a polyvalent metal cation crosslinking agent under conditions which gel the polymer, plugging off the more permeable zones, and diverting the subsequently injected water into the formerly less permeable but now relatively more permeable oil-bearing strata. The use of such as aluminum citrate and related polyvalent metal cations as crosslinking agents has been known for some time. However, citrate-sequestered aluminum type of agents has not been entirely satisfactory in some of the more hostile environments, such as formation temperatures of greater than about 170.degree. F. and/or hardness cation values in the formation water of greater than about 500 ppm.
Chromium(III) propionate solution is an effective agent for crosslinking polymers such as the partially hydrolyzed acrylamide-based polymers to form a gel in high permeability zones of an oil bearing reservoir, in either near-well treatments or indepth applications. Not only is chromium(III) propionate solution effective for such permeability correction processes, and is useful in hard brines, but that, surprisingly, the propionate component is biocidal against the pestiferous sulfate-reducing bacteria usually present in the formation water. Thus, both the crosslinking benefits can be obtained, plus the biocidal benefits.
However, preparation of useful chromium propionate solutions has proven difficult. The chromium(III) propionate solution must be discretely prepared. Sulfate by-products in the product solution, are undesirable because of the potential formation damage caused by precipitation of CaSO.sub.4 and/or BaSO.sub.4, and in addition can be metabolized by sulfate-reducing bacteria to produce corrosive "souring", gas H.sub.2 S. Some methods of preparation have resulted in sludges, objectionable due to potential clogging of the oil-bearing strata.
Aqueous solutions of sulfate-free propionate-sequestered chromium(III) of good clarity, easily and readily prepared in a reproducible manner have been a goal, or perhaps an obstacle, in the path of the use of this highly important material in oil field processes.